1. Field of the Invention
The present invention generally relates to methods for incorporating magnetic materials in a semiconductor manufacturing process and, more particularly, to methods for integrating a process of depositing magnetic materials during a semiconductor manufacturing process.
2. Background of the Invention
Magnetoelectronics is a growing field that is devoted to the development of electronic device structures that incorporate a ferromagnetic element. Magnetoelectronic devices are most-commonly used as storage devices by exploiting the bi-stable orientation characteristics of ferromagnetic material.
The bi-stable orientation characteristic is a defining characteristic of ferromagnetic materials and a natural basis for nonvolatile bit storage. A properly fabricated thin ferromagnetic film exhibits two possible states of magnetization that can be described by a hysteresis loop. As shown in FIG. 1, when a write current (Iw) is applied to an integrated, contiguous write wire 11 that is directly over a ferromagnetic element 12, a magnetic field (H) is generated that is parallel with and close to a surface of the write wire 11. The magnitude of the magnetic field (H) is determined by an inductive coefficient (α) and the write current (Iw), i.e., H=αIw. The magnetization of the ferromagnetic film is a function of the magnetic field and follows a hysteresis loop like that shown in FIG. 2.
More specifically, when the magnetic field is larger than a switching field (Hs), the magnetization of the ferromagnetic film reaches a first saturation value (Ms). The magnetization is thereafter maintained at this first saturation value and particular orientation for periods as long as years, even when power is removed. An output voltage (Vout) of a magnetic sensor in the proximity of the ferromagnetic material is also maintained unchanged until the magnetization is changed to a second saturation value (−Ms). The orientation of the magnetization changes when a magnetic field with a reversed direction is applied to the ferromagnetic element. The magnetization, however, drops down slightly when the reversed magnetic field is applied until the reversed magnetic field is less than −Hs. In this situation, the magnetization and output voltage jump promptly from the first saturation value (Ms, Vout) to the second saturation value (−Ms, −Vout), as shown in FIG. 2. As mentioned above, the magnetization state is maintained at the second saturation value for extremely long periods unless the magnetic field reaches Hs again.
Due to the “latched” or “non-volatile” characteristic of the ferromagnetic element described above, a ferromagnetic element can be maintain at its last state even after a power is removed from a write line. The ferromagnetic element can also be switched between two states in one clock cycle and the switch can be set and reset an infinite number of times. Generally, the switching speed of elements made from common ferromagnetic materials is on the order of a fraction of a nanosecond or even faster. In view of these performance characteristics, magnetoelectronic devices offer the promise of high speed, low power, and radiation hard nonvolatile magnetic memory and instantaneously programmable logic.
Utilization of a ferromagnetic element in an integrated magnetoelectronic device was first disclosed in an article entitled “Hybrid Hall Effect Device” by Mark Johnson et al. in 1997. In Johnson et al.'s prototype, a single microstructured ferromagnetic film and a micro scale Hall cross are fabricated together to create a magnetoelectronic device. Magnetic fringe fields from the edge of the ferromagnet generate a Hall voltage in the Hall cross. The sign of the fringe field, as well as the sign of the output Hall voltage, is switched by reversing the magnetization of the ferromagnet. The Hall cross thus detects the Hall voltage and outputs a value (high or low) corresponding to the direction of the magnetization of the ferromagnet.
Magnetoelectronic programmable logic to execute a Boolean operation can be devised utilizing Johnson's basic idea. FIG. 3 is a schematic diagram of an ideal magnetoelectronic programmable logic gate, which comprises two input terminals, A and B, one control terminal C, and separate terminals for bias and readout. Binary input pulses may be applied simultaneously to input terminals A and B, or the control terminal C. Each logic gate is constructed from a ferromagnetic material and associated Hall sensor. The ferromagnetic material is patterned from a thin magnetic film in such a way that a square hysteresis characteristic can be obtained. The up or down polarization of the magnetic material is detected by the Hall sensor as a high (“1”) or low (“0”) voltage. The logic gate is designed in a manner such that the saturation value of magnetization of the hysteresis loop can be reached only when at least two of the three terminals A, B and C are high inputs. Therefore, when a zero amplitude is applied to control terminal C(C=0), high inputs at terminals A and B are required to switch the gate state, and, accordingly, the logic gate operates as an AND gate. If a unit amplitude pulse is applied to C(C=1), then a single unit pulse at either A or B is necessary and sufficient to switch the gate state, and, accordingly, the logic gate operates as an OR gate. Two other Boolean functions, “NOR” and “NAND”, can be achieved by programming the bias current of the Hall sensor in an opposite direction. As mentioned, for a device like that shown in FIG. 3, Boolean operation requires only two clock steps for completion, one to set the initial device state and the second to sum input write currents. The result is then “latched” so that it can be read out at any later time.
FIG. 4 is a logic table showing that the programmable logic gate of FIG. 3 performs AND and NOR operations when C=1, and OR and NAND operation when C=0.
Exemplary electrical circuitry for achieving the logic gate of FIG. 3 is shown in FIG. 5, in which a VRESET voltage is used to reset the magnetization of a magnetic material to a known state. The logic operation starts with setting a set voltage, VSET, high. Then the input voltages VA, VB and VC switch parallel FETs X2-X8, X3-X7 and X4-X6 on or off, thus generating a write current in a metal strap that is positioned just above the magnetic material (not shown). The write current generates a magnetic field in the magnetic material and traverses the flux versus field hysteresis curve. The magnetic flux flows through active areas of the Hall sensor and generates voltages, corresponding to the vertical axis in FIG. 2. The Hall sensor thus detects the polarization of the magnetic material and outputs “1” or “0” in response to the detection.
In known magnetoelectronic devices, the ferromagnetic structures utilized to perform the bi-stable non-volatile function described have typically included giant mangetoresistance (GMR) structures and magnetic tunnel junctions (MTJ) structures. GMR structures are composed of all-metal ferromagnet-nonmagnetic-ferromagnet laminates that are typically low impedance devices, but require large bias current densities to achieve adequate output levels. MTJ structures are ferromagnet-insulator-ferromagnet structures with high impedance and bias-dependent output. In MTJ structures, output levels of tens of mV can be achieved with hundreds of mV bias, but the device impedance and output are extremely sensitive to the thickness of the insulator layer, and operation regions are difficult to control. Furthermore, GMR and MTJ structures both require two ferromagnetic films, and issues of magnetic coupling between layers impose further limits on their fabrication and yield of the resulting magnetoelectronic integrated circuits. Moreover, GMR and MTJ are distinct structures and cannot be readily manufactured during a semiconductor device manufacturing process.
As the competition in the magnetoelectronics market increases, there is a demand to reduce the volume of the ferromagnetic structure, simplify its manufacturing process, and better integrate it with a semiconductor manufacturing process.